Turbochargers are a type of forced induction system. They deliver compressed air to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. This can allow for the use of a smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, thus reducing the mass and aerodynamic frontal area of the vehicle. Turbochargers use the exhaust flow from the engine to drive a turbine, which in turn, drives the air compressor. At startup, the turbocharger may be at temperatures well below 0° C. Since the turbine spins at extremely high speed, in the range of 150,000 RPM to 300,000 RPM, is mechanically connected to the exhaust system, it sees high levels of temperature, up to 1050° C. for a gasoline engine, and vibration. Such conditions have a detrimental effect on the components of the turbocharger. Because of these adverse conditions the design, materials and tolerances must be selected to provide adequate life of the assembly for a given market cost. The design selections, required to satisfy these conditions, often lead to corrosion and resultant sticking of the VTG components.
Turbochargers, which utilize some form of turbine flow and pressure control are called by several names and offer control though various means. Some have rotating vanes; some have sliding sections or rings. Some titles for these devices are: variable turbine design (VTG), variable geometry turbine (VGT), variable nozzle turbine (VNT), or simply variable geometry (VG). The subject of this patent is the rotating vane type of variable turbine, which will be referred to as VTG for the remainder of this discussion.
Variable turbine geometry (VTG) turbochargers are known and utilize adjustable guide vanes rotatably connected to a vane ring or nozzle wall to control the amount of exhaust gas flow to the turbine wheel. The VTG turbochargers have a large number of components that must be assembled and positioned in the turbine housing so that the guide vanes are properly positioned with respect to the exhaust supply channel and the turbine wheel. VTG Turbochargers can employ an adjustment ring that actuates movement of the vanes for control of the amount of gas flow.
Exhaust gas is a high temperature corrosive gas. While it is known to make components of corrosion resistant materials such as high Chromium, high Vanadium steels, these materials are poor in machinability and are also expensive, thus unacceptably increase the material and manufacturing costs of a turbocharger turbine housing. Turbocharger turbine housings are most commonly made of cast iron, which is relatively inexpensive and easy to cast mold. However, cast iron is a highly porous material and is very susceptible to corrosion, particularly under conditions of high temperature cycling and exposure to diesel exhaust condensates. The effects of corrosion can spread to components that are in contact with the cast iron housing such as the adjustment ring. The adjustment ring, which requires relatively precise movement in order to actuate the vanes, can be prone to failure based upon corrosion from the turbine housing.
In U.S. Pat. No. 6,925,806 to Zollinger, applicants describe a typical adjustment or unison ring configuration as shown in FIG. 1. The variable geometry turbocharger assembly (38) has a plurality of vanes (40) movably disposed within the exhaust flow passage (42) of the turbine housing (44), and a post (46) is used to connect each vane to the nozzle wall (48). An annular unison ring (50) is positioned within the turbine housing (44), and is carried by a shoulder portion (54) of the center housing surface. The unison ring (50) is in direct contact with the center housing.
The center housing surface has a recessed portion (56) that extends radially outwardly a distance from the shoulder (54) and that provides an annular space (58) between an inside surface of the unison ring (50) and the center housing. The recessed portion (56) operates to provide a ventilation path for air to circulate behind the unison ring (50) to help control the unwanted accumulation of moisture therebetween. Moving radially inwardly from the shoulder portion (54), the center housing includes an annular heat shroud (60) that is attached thereto by a number of bolts (62). The heat shroud is a disk-shaped annular construction that is configured to fit over the region of the center housing surface extending roughly between the unison ring and a central shaft opening (64). The heat shroud operates to control the amount of heat transfer from the turbine housing to the central housing.
The Zollinger applicants correctly point out the drawback of the unison ring (50) being carried by, and in direct contact with, a portion of the center housing surface. The cast iron center housing in combination with moisture build-up between the unison ring (50) and center housing leads to corrosion in this area of the turbocharger. The corrosion can prevent proper unison ring rotational movement, thereby restricting or preventing desired vane operation
In U.S. Pat. No. 6,679,057 to Arnold, a turbine adjustment ring (19A) and a compressor adjustment ring (40A) are shown in FIG. 1A for a variable turbine and variable compressor geometry turbocharger. Both of the adjustment rings (19A) and (40A) are directly in contact with the housing where corrosion effects can lead to failure of the rings.
Referring again to U.S. Pat. No. 6,925,806, the Zollinger applicants attempt to prevent this problem of corrosion effects by providing a ring insert that is held in place by insert fasteners and which can be used to control movement of the unison ring. As shown in FIG. 2, the variable geometry turbocharger assembly (148) includes the unison ring insert (150) with unison ring insert fasteners (152) being used to connect the insert to the center housing surface, and an optional heat shroud (154) being held in place against the center housing by the insert. A recessed portion or annular space (156) between the center housing surface (158) and the inside surface (160) of the unison ring (162) is provided. An enlarged annular space (156) is provided in the form of a semi circular or “C”-shaped channel configured into the center housing surface. The enlarged annular space attempts to provide a further degree of gas circulation and ventilation between the unison ring (150) and the center housing to provide an enhanced amount of control over unwanted moisture accumulation therebetween.
The Zollinger unison ring insert (150) provides more complexity to an already complicated system and adds additional parts, and cost including the insert fasteners (152) and the heat shroud (154), which can be subject to failure in the stressful environment of the turbocharger. Additionally, because of the configuration of the unison ring insert (150) with the insert fasteners (152) and heat shroud (154) and their positioning with respect to the other components of the turbocharger, such as the vanes and vane posts, assembly of the turbocharger is made more difficult and time-consuming.
Thus, there is a need for a system and method for controlling movement of the adjustment or unison ring with respect to the center housing. There is a further need for such a system and method that reduces and/or prevents corrosion effects to the assembly. There is a further need for such a system and method that accounts for thermal growth of the components while maintaining efficiencies. There is a yet a further need for such a system and method that is cost effective and dependable. There is additionally a need for such a system and method that facilitates manufacture, assembly and/or disassembly.